From U.S. Pat. No. 3,468,164 an open thermocouple detection circuit has become known. Said open thermocouple detection circuit provides a continuous test signal to selected thermocouples to be tested. In the case of a faulty or open thermocouple the full test signal, having an opposite polarity compared to the normal thermocouple output signal, is applied to the output signal sensing device. A decoupling circuit is operative with a test signal oscillator transformer to prevent common mode voltage and offset current flow through the test signal supply to ground.
However, not only the case of an open thermocouple may appear but the thermo wires of a thermocouple may also be short circuit, e.g. by mechanical bending or by loss of the thermal insulation between the thermo wires due to high temperature or aging of the thermo wires.
U.S. Pat. No. 3,671,953 discloses an alarm annunciator having hot, cold and thermocouple burn-out capability without requiring specific resistor connections for the hot and cold alarm modes of operation. An alarm burn-out switch responds to certain voltage variations in the annunciator signal translating circuitry during alarm thermocouple burn-out to provide electrical control of an alarm indicating means. The burn-out switch also serves as a constant current sink during both the hot and cold alarm modes of operation.
From U.S. Pat. No. 4,211,113 a temperature measurement has become known in which a thermocouple and a reference voltage source in turn supply voltages via solid state switches to the center-tapped primary winding of a transformer. The output voltages of the transformer secondary winding are applied via an analogue-to-digital converter to a crystal-clock-regulated digital processing unit that obtains the difference of positive and negative voltages derived from the thermocouple signal, the positive and negative voltages derived from the reference voltage, for subsequent ratiometric processing.
From “RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Family of Devices”, Application Report, SBAA201-March 2013, Texas Instruments devices that are optimized for that are optimized for the measurement of temperature sensors have become known. Thus, in a typical RTD measurement application, the ADS1148 and ADS1248 are configured in a ratiometric topology using a built-in IDAC (current-Digital-to-Analog-Converter) current source feeding through an external reference precision resistor. However, no reliable diagnosing a thermocouple's condition, e.g. the degree of degradation of the thermo wires forming the thermo couple, has become known from the prior art.